In modern communications systems, phase locked loops used for direct carrier frequency modulation require accurate knowledge of the controlled oscillator gain, particularly when the modulation signal shows a large frequency bandwidth. The reason for this is that when the modulation signal has a larger bandwidth than the PLL loop bandwidth, methods to increase the modulation bandwidth of the PLL are generally implemented. These methods require an accurate knowledge of the oscillator gain.
One method to increase the modulation bandwidth of a PLL is pre-emphasis of the modulation signal, in order to compensate for the lowpass characteristic of the PLL transfer function. This method works well for narrowband modulation schemes such as GSM. To be able to compensate the loop transfer function, the open loop gain has to be known with an accuracy of about 5%.
A more advanced wideband modulation method is 2-point modulation. This method utilizes two additive modulation paths, where one has a highpass transfer function and the second one a lowpass transfer function. The sum of both paths is ideally not limited anymore in bandwidth (except for very high frequency offsets). For a good matching between the two paths, the oscillator gain should generally be known to within an allowed deviation of about 1.5-2%. However, the oscillator gain of fully integrated PLLs varies much more than these values, and obtaining the required gain measurement accuracy is difficult to achieve, particularly with the non-linear oscillator control characteristic.
Accordingly, there is a continued need to improve PLL based frequency modulators having accurate gain control.